Supercapacitors (Ultra-Capacitors or Electro-Chemical Capacitors):
The high volumetric capacitance density of a supercapacitor derives from using porous electrodes to create a large surface area conducive to the formation of diffuse electric double layer (EDL) charges. The ionic species (cations and anions) in the EDL are formed in the electrolyte near an electrode surface (but not on the electrode surface per se) when voltage is imposed upon a symmetric supercapacitor (or EDLC), as schematically illustrated in FIG. 1(A). The required ions for this EDL mechanism pre-exist in the liquid electrolyte (randomly distributed in the electrolyte) when the cell is made or in a discharged state (FIG. 1(B)). These ions do not come from the opposite electrode material. In other words, the required ions to be formed into an EDL near the surface of a negative electrode (anode) active material (e.g., activated carbon particle) do not come from the positive electrode (cathode); i.e., they are not previously captured or stored in the surfaces or interiors of a cathode active material. Similarly, the required ions to be formed into an EDL near the surface of a cathode active material do not come from the surface or interior of an anode active material.
When the supercapacitor is re-charged, the ions (both cations and anions) already pre-existing in the liquid electrolyte are formed into EDLs near their respective local electrodes. There is no exchange of ions between an anode active material and a cathode active material. The amount of charges that can be stored (capacitance) is dictated solely by the concentrations of cations and anions that pre-exist in the electrolyte. These concentrations are typically very low and are limited by the solubility of a salt in a solvent, resulting in a low energy density.
In some supercapacitors, the stored energy is further augmented by pseudo-capacitance effects due to some electrochemical reactions (e.g., redox). In such a pseudo-capacitor, the ions involved in a redox pair also pre-exist in the electrolyte. Again, there is no exchange of ions between an anode active material and a cathode active material.
Since the formation of EDLs does not involve a chemical reaction or an exchange of ions between the two opposite electrodes, the charge or discharge process of an EDL supercapacitor can be very fast, typically in seconds, resulting in a very high power density (more typically 3,000-8,000 W/Kg). Compared with batteries, supercapacitors offer a higher power density, require no maintenance, offer a much higher cycle-life, require a very simple charging circuit, and are generally much safer. Physical, rather than chemical, energy storage is the key reason for their safe operation and extraordinarily high cycle-life.
Despite the positive attributes of supercapacitors, there are several technological barriers to widespread implementation of supercapacitors for various industrial applications. For instance, supercapacitors possess very low energy densities when compared to batteries (e.g., 5-8 Wh/kg for commercial supercapacitors vs. 20-30 Wh/Kg for the lead acid battery and 50-100 Wh/kg for the NiMH battery). Lithium-ion batteries possess a much higher energy density, typically in the range of 100-180 Wh/kg, based on the total cell weight.
Lithium-Ion Batteries (LIB):
Although possessing a much higher energy density, lithium-ion batteries deliver a very low power density (typically 100-500 W/Kg), requiring typically hours for re-charge. Conventional lithium-ion batteries also pose some safety concern.
The low power density or long re-charge time of a lithium ion battery is due to the mechanism of shuttling lithium ions between the interior of an anode and the interior of a cathode, which requires lithium ions to enter or intercalate into the bulk of anode active material particles during re-charge, and into the bulk of cathode active material particles during discharge. For instance, as illustrated in FIG. 1(C), in a most commonly used lithium-ion battery featuring graphite particles as an anode active material, lithium ions are required to diffuse into the inter-planar spaces of a graphite crystal at the anode during re-charge. Most of these lithium ions have to come all the way from the cathode side by diffusing out of the bulk of a cathode active particle, through the pores of a solid separator (pores being filled with a liquid electrolyte), and into the bulk of a graphite particle at the anode.
During discharge, lithium ions diffuse out of the anode active material (e.g. de-intercalate out of graphite particles 10 μm in diameter), migrate through the liquid electrolyte phase, and then diffuse into the bulk of complex cathode crystals (e.g. intercalate into particles lithium cobalt oxide, lithium iron phosphate, or other lithium insertion compound), as illustrated in FIG. 1(D). Because liquid electrolyte only reaches the external surface (not interior) of a solid particle (e.g. graphite particle), lithium ions swimming in the liquid electrolyte can only migrate (via fast liquid-state diffusion) to the surface of a graphite particle. To penetrate into the bulk of a solid graphite particle would require a slow solid-state diffusion (commonly referred to as “intercalation”) of lithium ions. The diffusion coefficients of lithium in solid particles of lithium metal oxide are typically 10−16-10−8 cm2/sec (more typically 10−14-10−10 cm2/sec), and those of lithium in liquid are approximately 10−6 cm2/sec.
In other words, these intercalation or solid-state diffusion processes require a long time to accomplish because solid-state diffusion (or diffusion inside a solid) is difficult and slow. This is why, for instance, the current lithium-ion battery for plug-in hybrid vehicles requires 2-7 hours of recharge time, as opposed to just seconds for supercapacitors. The above discussion suggests that an energy storage device that is capable of storing as much energy as in a battery and yet can be fully recharged in one or two minutes like a supercapacitor would be considered a revolutionary advancement in energy storage technology.
Lithium Ion Capacitors (LIC):
A hybrid energy storage device that is developed for the purpose of combining some features of an EDL supercapacitor (or symmetric supercapacitor) and those of a lithium-ion battery (LIB) is a lithium-ion capacitor (LIC). A LIC contains a lithium intercalation compound (e.g., graphite particles) as an anode and an EDL capacitor-type cathode (e.g. activated carbon, AC), as schematically illustrated in FIG. 1(E). In a commonly used LIC, LiPF6 is used as an electrolyte salt, which is dissolved in a solvent, such as propylene carbonate. When the LIC is in a charged state, lithium ions are retained in the interior of the lithium intercalation compound anode (usually micron-scaled graphite particles) and their counter-ions (e.g. negatively charged PF6−) are disposed near activated carbon surfaces (but not on an AC surface, or captured by an AC surface), as illustrated in FIG. 1(E).
When the LIC is discharged, lithium ions migrate out from the interior of graphite particles (a slow solid-state diffusion process) to enter the electrolyte phase and, concurrently, the counter-ions PF6− are also released from the EDL zone, moving further away from AC surfaces into the bulk of the electrolyte. In other words, both the cations (Li+ ions) and the anions (PF6−) are randomly disposed in the liquid electrolyte, not associated with any electrode (FIG. 1(F)). This implies that, just like in a symmetric supercapacitor, the amounts of both the cations and the anions that dictate the specific capacitance of a LIC are essentially limited by the solubility limit of the lithium salt in a solvent (i.e. limited by the amount of LiPF6 that can be dissolved in the solvent). Therefore, the energy density of LICs (a maximum of 14 Wh/kg) is not much higher than that (6 Wh/kg) of an EDLC (symmetric supercapacitor), and remains an order of magnitude lower than that (most typically 120-150 Wh/kg) of a LIB.
Furthermore, due to the need to undergo de-intercalation and intercalation at the anode, the power density of a LIC is not high (typically <15 kW/kg, which is comparable to or only slightly higher than those of an EDLC).
The above review of the prior art indicates that a battery has a higher energy density, but is incapable of delivering a high power (high currents or pulse power) that an EV, HEV, or micro-EV needs for stop/start and accelerating. A battery alone is also not capable of capturing and storing the braking energy of a vehicle. A supercapacitor or LIC can deliver a higher power, but does not store much energy (the stored energy only lasts for a short duration of operating time) and, hence, cannot be a single power source alone to meet the energy/power needs of an EV or HEV.
More Recent Developments:
Most recently, our research group has invented a revolutionary class of high-power and high-energy-density energy storage devices now commonly referred to as the surface-mediated cell (SMC). This has been reported in the following patent applications and a scientific paper:                1. C. G. Liu, et al., “Lithium Super-battery with a Functionalized Nano Graphene Cathode,” U.S. patent application Ser. No. 12/806,679 (Aug. 19, 2010).        2. C. G. Liu, et al, “Lithium Super-battery with a Functionalized Disordered Carbon Cathode,” U.S. patent application Ser. No. 12/924,211 (Sep. 23, 2010).        3. Aruna Zhamu, C. G. Liu, David Neff, and Bor Z. Jang, “Surface-Controlled Lithium Ion Exchanging Energy Storage Device,” U.S. patent application Ser. No. 12/928,927 (Dec. 23, 2010).        4. Aruna Zhamu, C. G. Liu, David Neff, Z. Yu, and Bor Z. Jang, “Partially and Fully Surface-Enabled Metal Ion-Exchanging Battery Device,” U.S. patent application Ser. No. 12/930,294 (Jan. 3, 2011).        5. Aruna Zhamu, Chen-guang Liu, X. Q. Wang, and Bor Z. Jang, “Surface-Mediated Lithium Ion-Exchanging Energy Storage Device,” U.S. patent application Ser. No. 13/199,450 (Aug. 30, 2011).        6. Aruna Zhamu, Chen-guang Liu, and Bor Z. Jang, “Partially Surface-Mediated Lithium Ion-Exchanging Cells and Method of Operating Same,” U.S. patent application Ser. No. 13/199,713 (Sep. 7, 2011).        7. Bor Z. Jang, C. G. Liu, D. Neff, Z. Yu, Ming C. Wang, W. Xiong, and A. Zhamu, “Graphene Surface-Enabled Lithium Ion-Exchanging Cells: Next-Generation High-Power Energy Storage Devices,” Nano Letters, 2011, 11 (9), pp 3785-3791.        
There are two types of SMCs: partially surface-mediated cells (p-SMC, also referred to as lithium super-batteries) and fully surface-mediated cells (f-SMC). Both types of SMCs contain the following components:                (a) An anode containing an anode current collector (such as copper foil) in a lithium super-battery or p-SMC, or an anode current collector plus an anode active material in an f-SMC. The anode active material in an f-SMC is preferably a nano-carbon material (e.g., graphene) having a high specific surface area (preferably >100 m2/g, more preferably >500 m2/g, further preferably >1,000 m2/g, and most preferably >1,500 m2/g). The anode current collector is preferably porous, having large surface areas to accommodate returning lithium ions when the cell is being re-charged;        (b) A cathode containing a cathode current collector and a cathode active material (e.g. graphene or disordered carbon) having a high specific surface area (preferably >100 m2/g, more preferably >500 m2/g, further preferably >1,000 m2/g, still more preferably >1,500 m2/g, and most preferably >2,000 m2/g);        (c) A porous separator separating the anode and the cathode, soaked with an electrolyte (preferably liquid or gel electrolyte); and        (d) A lithium source disposed in an anode or a cathode (or both) and in direct contact with the electrolyte.        
In a fully surface-mediated cell, f-SMC, as illustrated in FIG. 2, both the cathode active material and the anode active material are porous, having large amounts of graphene surfaces in direct contact with liquid electrolyte. These electrolyte-wetted surfaces are ready to interact with nearby lithium ions dissolved therein, enabling fast and direct adsorption of lithium ions on graphene surfaces and/or redox reaction between lithium ions and surface functional groups, thereby removing the need for solid-state diffusion or intercalation. When the SMC cell is made, particles or foil of lithium metal are implemented at the anode (FIG. 2A), which are ionized during the first discharge cycle, supplying a large amount of lithium ions. These ions migrate to the nano-structured cathode through liquid electrolyte, entering the pores and reaching the surfaces in the interior of the cathode without having to undergo solid-state intercalation (FIG. 2B). When the cell is re-charged, a massive flux of lithium ions are quickly released from the large amounts of cathode surfaces, migrating into the anode zone. The large surface areas of the nano-structured anode enable concurrent and high-rate deposition of lithium ions (FIG. 2C), re-establishing an electrochemical potential difference between the lithium-decorated anode and the cathode.
A particularly useful nano-structured electrode material is nano graphene platelet (NGP), which refers to either a single-layer graphene sheet or multi-layer graphene pletelets. A single-layer graphene sheet is a 2-D hexagon lattice of carbon atoms covalently bonded along two plane directions. We have studied a broad array of graphene materials for electrode uses: pristine graphene, graphene oxide, chemically or thermally reduced graphene, graphene fluoride, chemically modified graphene, hydrogenated graphene, nitrogenated graphene, doped graphene. In all cases, both single-layer and multi-layer graphene were prepared from natural graphite, petroleum pitch-derived artificial graphite, micron-scaled graphite fibers, activated carbon (AC), and treated carbon black (t-CB). AC and CB contain narrower graphene sheets or aromatic rings as a building block, while graphite and graphite fibers contain wider graphene sheets. Their micro-structures all have to be exfoliated (to increase inter-graphene spacing in graphite) or activated (to open up nano gates or pores in t-CB) to allow liquid electrolyte to access more graphene edges and surfaces where lithium can be captured. Other types of disordered carbon studied have included soft carbon (including meso-phase carbon, such as meso-carbon micro-beads), hard carbon (including petroleum coke), and amorphous carbon, in addition to carbon black and activated carbon. All these carbon/graphite materials have graphene sheets dispersed in their microstructure.
These highly conducting materials, when used as a cathode active material, can have a functional group that is capable of rapidly and reversibly forming a redox reaction with lithium ions. This is one possible way of capturing and storing lithium directly on a graphene surface (including edge). We have also discovered that the benzene ring centers of graphene sheets are highly effective and stable sites for capturing and storing lithium atoms, even in the absence of a lithium-capturing functional group.
Similarly, in a lithium super-battery (p-SMC), the cathode includes a chemically functionalized NGP or a functionalized disordered carbon material having certain specific functional groups capable of reversibly and rapidly forming/releasing a redox pair with a lithium ion during the discharge and charge cycles of a p-SMC. In a p-SMC, the disordered carbon or NGP is used in the cathode (not the anode) of the lithium super-battery. In this cathode, lithium ions in the liquid electrolyte only have to migrate to the edges or surfaces of graphene sheets (in the case of functionalized NGP cathode), or the edges/surfaces of the aromatic ring structures (small graphene sheets) in a disordered carbon matrix. No solid-state diffusion is required at the cathode. The presence of a functionalized graphene or carbon having functional groups thereon enables reversible storage of lithium on the surfaces (including edges), not the bulk, of the cathode material. Such a cathode material provides one type of lithium-storing or lithium-capturing surface. Again, another possible mechanism is based on the benzene ring centers of graphene sheets that are highly effective and stable sites for capturing and storing lithium atoms.
In a lithium super-battery or p-SMC, the anode comprises a current collector and a lithium foil alone (as a lithium source), without an anode active material to support or capture lithium ions/atoms. Lithium has to deposit onto the front surface of an anode current collector alone (e.g. copper foil) when the battery is re-charged. Since the specific surface area of a current collector is very low (typically <1 m2/gram), the over-all lithium re-deposition rate can be relatively low as compared to f-SMC.
The features and advantages of SMCs that differentiate the SMC from conventional lithium-ion batteries (LIB), supercapacitors, and lithium-ion capacitors (LIC) are summarized below:                (A) In an SMC, lithium ions are exchanged between anode surfaces and cathode surfaces, not bulk or interior:                    a. The conventional LIB stores lithium in the interior of an anode active material (e.g. graphite particles) in a charged state (e.g. FIG. 1(C)) and the interior of a cathode active material in a discharged state (FIG. 1(D)). During the discharge and charge cycles of a LIB, lithium ions must diffuse into and out of the bulk of a cathode active material, such as lithium cobalt oxide (LiCoO2) and lithium iron phosphate (LiFePO4). Lithium ions must also diffuse in and out of the inter-planar spaces in a graphite crystal serving as an anode active material. The lithium insertion or extraction procedures at both the cathode and the anode are very slow, resulting in a low power density and requiring a long re-charge time.            b. When in a charged state, a LIC also stores lithium in the interior of graphite anode particles (FIG. 1(E)), thus requiring a long re-charge time as well. During discharge, lithium ions must also diffuse out of the interior of graphite particles, thereby compromising the power density. The lithium ions (cations Li+) and their counter-ions (e.g. anions PF6−) are randomly dispersed in the liquid electrolyte when the LIC is in a discharged state (FIG. 1(F)). In contrast, the lithium ions are captured by graphene surfaces (e.g. at centers of benzene rings of a graphene sheet as illustrated in FIG. 2(D)) when an SMC is in a discharged state. Lithium is deposited on the surface of an anode (anode current collector and/or anode active material) when the SMC is in a charged state. Relatively few lithium ions stay in the liquid electrolyte.            c. When in a charged state, a symmetric supercapacitor (EDLC) stores their cations near a surface (but not at the surface) of an anode active material (e.g. activated carbon, AC) and stores their counter-ions near a surface (but not at the surface) of a cathode active material (e.g., AC), as illustrated in FIG. 1(A). When the EDLC is discharged, both the cations and their counter-ions are re-dispersed randomly in the liquid electrolyte, further away from the AC surfaces (FIG. 1(B)). In other words, neither the cations nor the anions are exchanged between the anode surface and the cathode surface.            d. For a supercapacitor exhibiting a pseudo-capacitance or redox effect, either the cation or the anion form a redox pair with an electrode active material (e.g. polyanniline or manganese oxide coated on AC surfaces) when the supercapacitor is in a charged state. However, when the supercapacitor is discharged, both the cations and their counter-ions are re-dispersed randomly in the liquid electrolyte, away from the AC surfaces. Neither the cations nor the anions are exchanged between the anode surface and the cathode surface. In contrast, the cations (Li+) are captured by cathode surfaces (e.g. graphene benzene ring centers) when the SMC is in the discharged state. It is also the cations (Li+) that are captured by surfaces of an anode current collector and/or anode active material) when the SMC is in the discharged state. In other words, the lithium ions are shuttled between the anode surfaces and the cathode surfaces.            e. An SMC operates on the exchange of lithium ions between the surfaces of an anode (anode current collector and/or anode active material) and a cathode (cathode active material). The cathode in a SMC has (a) benzene ring centers on a graphene plane to capture and release lithium; (b) functional groups (e.g. attached at the edge or basal plane surfaces of a graphene sheet) that readily and reversibly form a redox reaction with a lithium ion from a lithium-containing electrolyte; and (c) surface defects to trap and release lithium during discharge and charge. Unless the cathode active material (e.g. graphene, CNT, or disordered carbon) is heavily functionalized, mechanism (b) does not significantly contribute to the lithium storage capacity.                            When the SMC is discharged, lithium ions are released from the surfaces of an anode (surfaces of an anode current collector and/or surfaces of an anode active material, such as graphene). These lithium ions do not get randomly dispersed in the electrolyte. Instead, these lithium ions swim through liquid electrolyte and get captured by the surfaces of a cathode active material. These lithium ions are stored at the benzene ring centers, trapped at surface defects, or captured by surface/edge-borne functional groups. Very few lithium ions remain in the liquid electrolyte phase.                When the SMC is re-charged, massive lithium ions are released from the surfaces of a cathode active material having a high specific surface area. Under the influence of an electric field generated by an outside battery charger, lithium ions are driven to swim through liquid electrolyte and get captured by anode surfaces, or are simply electrochemically plated onto anode surfaces.                                                (B) In a discharged state of a SMC, a great amount of lithium atoms are captured on the massive surfaces of a cathode active material. These lithium ions in a discharged SMC are not dispersed or dissolved in the liquid electrolyte, and not part of the electrolyte. Therefore, the solubility limit of lithium ions and/or their counter-ions does not become a limiting factor for the amount of lithium that can be captured at the cathode side. It is the specific surface area at the cathode that dictates the lithium storage capacity of an SMC provided there is a correspondingly large amount of available lithium atoms at the lithium source prior to the first discharge/charge.        (C) During the discharge of an SMC, lithium ions coming from the anode side through a separator only have to diffuse in the liquid electrolyte residing in the cathode to reach a surface/edge of a graphene plane. These lithium ions do not need to diffuse into or out of the volume (interior) of a solid particle. Since no diffusion-limited intercalation is involved at the cathode, this process is fast and can occur in seconds. Hence, this is a totally new class of energy storage device that exhibits unparalleled and unprecedented combined performance of an exceptional power density, high energy density, long and stable cycle life, and wide operating temperature range. This device has exceeded the best of both battery and supercapacitor worlds.        (D) In an f-SMC, the energy storage device operates on lithium ion exchange between the cathode and the anode. Both the cathode and the anode (not just the cathode) have a lithium-capturing or lithium-storing surface and both electrodes (not just the cathode) obviate the need to engage in solid-state diffusion. Both the anode and the cathode have large amounts of surface areas to allow lithium ions to deposit thereon simultaneously, enabling dramatically higher charge and discharge rates and higher power densities.                    The uniform dispersion of these surfaces of a nano-structured material (e.g. graphene, CNT, disordered carbon, nano-wire, and nano-fiber) at the anode also provides a more uniform electric field in the electrode in which lithium can more uniformly deposit without forming a dendrite. Such a nano-structure eliminates the potential formation of dendrites, which was the most serious problem in conventional lithium metal batteries (commonly used in 1980s and early 1990s before being replaced by lithium-ion batteries).                        (E) A SMC typically has an open-circuit voltage of >1.0 volts (most typically >1.5 volts) and can operate up to 4.5 volts for lithium salt-based organic electrolyte. Using an identical electrolyte, an EDLC or symmetric supercapacitor has an open-circuit voltage of essentially 0 volts and can only operate up to 2.7 volts. Also using an identical electrolyte, a LIC operates between 2.2 volts and 3.8 volts. These are additional manifestations of the notion that the SMC is fundamentally different and patently distinct from both an EDLC and a LIC.        
The amount of lithium stored in the lithium source when a SMC is made dictates the amount of lithium ions that can be exchanged between an anode and a cathode. This, in turn, dictates the energy density of the SMC.
Upon further studies, we have made several significant yet unexpected discoveries. First, we have observed that a SMC can operate at a lower voltage limit (LVL) that is lower than the open-circuit voltage (OCV) of the cell when it is made. We have surprisingly observed that by operating the SMC down to a lower voltage limit (LVL<OCV) enables us to achieve a significantly higher cathode capacity and higher energy density. We have further discovered that carbon aerogel and related carbon gel or foam materials, including graphene-enhanced carbon aerogel and CNT-enhanced carbon aerogel, are outstanding SMC electrode materials, providing exceptionally high energy and power density values.